Optical systems employed in the field of EUV lithography, such as illumination systems and projection systems in a EUV lithography apparatus, require reflective optical elements providing relatively high reflectivity for electromagnetic radiation from the extreme ultraviolet (EUV) wavelength range. The EUV wavelength range includes wavelengths between about 5 nm and 20 nm, for example. The lower end of this wavelength range is sometimes also denoted as beyond EUV (BEUV) or soft X-ray range. The optical elements employed in a EUV lithography system include curved or plane mirrors, reflective photo masks and other reflective elements capable of guiding or structuring EUV radiation. Reflective optical elements are also needed in optical inspection systems used to inspect photomasks or other patterning arrangements using EUV radiation, or in EUV microscopes.
Any material used to reflect radiation in the EUV or soft x-ray range reflects only a few percents of incident radiation because the real part of the refractive index is close to unity. The complex refractive index n of a material may be described as a sum of the real part (1−δ) and the imaginary part iß of the refractive index in accordance with n=(1−δ)+iß. In this notation, the dimensionless parameter δ describes the deviation of the real part of the refractive index n from the value 1 and may be denoted as “refractive index decrement”. The dimensionless parameter ß represents the commonly used extinction coefficient k. Differences in the real part of the refractive index at an interface between adjacent materials govern the contribution to reflectivity of the interface. Those differences (Δδ) are sometimes referred to as “refractive index contrast”. The magnitude of the extinction coefficient is indicative of the amount of absorption of EUV radiation in a material and includes scattering properties on atomic level.
Due to the fact that only small differences in the real part of refractive index exist in the EUV or soft X-ray range, a reflective optical element operating at normal incidence or near normal incidence of EUV radiation must be designed as a multilayer mirror in order to be capable of reflecting EUV radiation with reasonably high reflectivity, for example with reflectivity of 20% or more.
In this application, the term “multilayer mirror” generally refers to any reflective optical element having multiple material layers effective to reflect EUV or soft X-ray radiation.
A multilayer mirror may be described as comprising a substrate and a stack of layers formed on the substrate, wherein the stack of layers comprises layers comprising a “low index material” and “high index material”, the low index material having a lower real part of the refractive index than the high index material at a given operating wavelength λ in the EUV range.
It is important to note that the terms “low index material” and “high index material” do not describe absolute properties of the materials. Instead, “high” and “low” denote properties relative to adjacent layers. A specific first material may be a “low index material” relative to a specific second material, while the same first material may be a “high refractive index material relative to a third material different from the first material.
The material having a relatively lower real part of the refractive index at the operating wavelength (i.e. the low index material) is commonly denoted by the capital letter “L”, while the material having a relatively higher real part of the refractive index (i.e. the high index material) at the operating wavelength is commonly denoted by the capital letter “H”.
Multilayer mirrors typically include a plurality of repetitive units stacked upon each other in a recurring sequence. A repetitive unit including a single low index material layer and a single adjacent high index material layer may be referred to as “bilayer” or two-layer repetitive unit. A repetitive unit may include one or more additional layers, for example a diffusion barrier interlayer interposed between a low index material layer and a high index material layer to reduce intermixing of the adjacent materials at the interface. A sequence of repetitive units in a multilayer mirror forms a stack of layers which essentially simulates a crystal partially reflecting the incident radiation at interfaces between adjacent layers. The thicknesses of the individual layers as well as the total thickness of the repetitive units can be constant across the entire multilayer system, or they can vary laterally and/or in depth depending on which reflection profile is to be achieved.
A specific problem in the EUV region is that all useful materials absorb the radiation to a certain extent. To reduce the effect of absorption losses on reflectivity, the geometrical thicknesses of the two components of the bilayer (low and high index material) are typically adjusted to deviate slightly from the optical path length of λ/4, where λ is the design wavelength. Specifically, the thickness of the high absorption layer, dh, (i.e. the thickness of the material having the higher extinction coefficient k) is often set slightly lower than the thickness dl of the low absorption layer. A partition ratio Γ=dh/d of the multilayer is typically defined to describe this approach. The partition ratio is defined as the ratio of the thickness of the material having the higher extinction coefficient, k, to the total thickness d of the period, also denoted as “period thickness”. Partition ratios in the order of Γ=0.4 are commonly used.
Note that the material with the higher extinction coefficient (indicated by lower case letter “h”) may be the material having the relatively lower real part of refractive index (indicated by capital letter “L”), and vice versa.
An optimum thickness of a repetitive unit may be calculated for a given wavelength and a given range of angles of incidence using Bragg's law.
According to Singh (e.g. US 2003/0043456 A1) or M. Singh and J. J. M. Braat: “Improved Theoretical Reflectivities of Extreme Ultraviolet Mirrors” in: Proc. SPIE, 3997, 412-419 (2000) two designs predominate for the 11-16 nm wavelength spectral region: Mo/Be for the 11.3 nm window consisting typically of 80 periods and the Mo/Si system for the 13.4 nm window, typically consisting of 40-50 periods. In each multilayer system, Molybdenum (Mo) forms the low index material layers, while Beryllium (Be) and Silicon (Si) form the respective high index material layers. These designs yield maximum theoretical reflectivities of R≈0.78 for the Mo/Be multilayer mirror, and R≈0.74 for the Mo/Si multilayer mirror while taking into account a highly absorbing native oxide layer with about 2 nm thickness on the surface Si layer.
Singh reports that the reflectivity of multilayered EUV mirrors tuned for 11-16 nm, for which the two-component Mo/Be and Mo/Si multilayered systems were previously used, may be enhanced by incorporating additional elements and their compounds mainly from period 5 of the periodic table. In addition, the reflectivity performance of the multilayer mirrors may be further enhanced by a numerical global optimization procedure by which the layer thicknesses are varied for optimum performance in contradistinction to previous constant layer thickness (i.e. constant partition ratio) multilayer stacks. By incorporating additional materials with differing complex refractive indices in various regions of a stack, or by wholly replacing one of the components (typically Mo), Singh observed peak reflectivity enhancements of up to 5% for a single mirror compared to a standard unoptimized stack. Rb, RbCl, Sr, Y, Zr, Ru, Rh, Tc, Pd, Nb and Be were used as the additional materials. Protective capping layers of B, Ru, Rh, C, Si3N4, SiC, in addition to protecting the mirrors from environmental attack, may serve to improve the reflectivity characteristics.
Besides various attempts to increase maximum reflectivity there have also been many attempts to increase spectral and/or angular bandwidth of multilayer mirrors (see e.g. US 2005/0111083 A1)
It has further been proposed that radiation with a wavelength of less than 10 nm could be used in microlithograpy, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as “beyond EUV”, or as “soft x-rays”. However, in the context of the present application, this wavelength range is encompassed by the term “EUV radiation”.
To reflect EUV radiation having a wavelength of about 6.7 nm, multilayer mirrors have been proposed having alternating layers of a metal, such as La, U or Th, and B or a B compound, such as B4C or B9C. Such a multilayer mirror reflects the EUV radiation according to Bragg's Law. However, chemical interaction of for instance La and the B layer or the B compound layer may lead to interlayer diffusion (see e.g. WO 2010/091907).
US 2003/0185341 A1 suggests multilayer mirrors for wavelengths of about 6.64 nm having a multilayer structure consisting of at least a first layer of a lanthanum (La) containing compound and at least a second layer of a boron (B) containing compound alternately disposed on a substrate.
According to US 2011/0194087 A1 the maximum reflectivity theoretically achievable by multilayer systems for operating wavelengths in the wavelength range between 5 nm and 12 nm is smaller than in the wavelength range from about 12 nm to 20 nm. Also, the bandwidth of the reflected radiation is substantially smaller. Problems of interlayer diffusion are addressed, for example in multilayer mirrors where lanthanum is used as a low index material and boron or boron carbide is used as a high index material. As a solution it is proposed that a further layer of a nitride or a carbide of the material having the lower real part of the refractive index is arranged at an interface from the material having the higher real part of the refractive index to the material having the lower real part of the refractive index. The material having the lower real part of the refractive index may be lanthanum or thorium, for example.